1. Field of the Invention
The present invention relates primarily to an improved prosthesis of unicompartmental total knee replacement to both the femur and to the tibial, and more specifically to a special dove tail extension from the prosthesis.
2. Brief Description of the Prior Art
Resurfacing or replacement of one component of the knee is not a new concept. It is widely recognized that there are three components to the knee, the medial compartment, lateral compartment and the patellofemoral compartment. "Tibial Plateau Prosthesis," by McKeever, D. C. (Clin. Othrop., 18:86-95, 1960) and "The use of Hemiarthroplasty Prosthesis for Advanced Osteoarthritic and Rheumatoid arthritis of the knee," by MacIntosh, D. I. and Hunter, G. A. (J. Bone Joint Surgery, 54-B(2): 244-255, 1972) reported on the insertion of the prosthetic disc into the worn out tibial plateau "Unicompartmental Knee Replacement," by Marmor, L. (In Total Knee Arthroplasty, pp. 245-280. Edited by J. A. Rand. New York, Raven Press, 1993). Early total knee designs consisted of constraining unicompartmental replacements and preceded the accepted form of the total knee as now known as a Duocondylar total prosthesis as described in "Historical Development, Classification, and Charachteristics of Knee Prostheses," by Insall, J. N. (In Surgery of the Knee, pp. 677-717. Edited by J. N. Insall. New York, Chruchill Livingstone, 1993).
Through the unicompartmental design described in "Die Schlittnprothese, eine Teiiprothese bei Zerstorungen im Kniegelenk," by Engelbrecht, E. (Chirurg, 42: 510-514, 1971) added a true polycentric femoral component and "Unicompartmental Knee Replacement," by Marmor, L. (Total Knee Arthroplasty pp. 245-280. Edited by J. A. Rand. New York, Raven Press, 1993) added a so called modularity to the tibia component in the sense of providing different plateau thicknesses. "Clinical Results of the Oxford Knee. Surface arthroplasty of the Tibiofomoral Joint with a Meniscal Bearing Prosthesis," by Goodfellow, J. W. and O'Connor, J. (Clin. Orthop., 205:21-42, 1988) provided a so called meniscal bearing plateau. For the last twenty years, candidates for the unicompartmental knee have done well with this prosthesis, however breakage of the prosthesis and early wear of the bearing surface have caused many surgeons to avoid the unicompartmental replacement, instead doing a complete total knee, even though only one of the three joints of the knee was involved. It has been found that the bone on the effected side, usually the medial side of the joint is very thick and hardened with eburnation and this may be the hardest bone in the body. With this very hard bone, it is felt that minimal resection would keep the prosthesis from subsiding or sinking into the softer bone beneath, and for that reason as little bone as possible is resected from both the tibial and the femur at the time of insertion of the prosthesis.
The designs of the unicompartmental replacement do very well. The Marmor knee prosthesis which was introduced in the early seventies, featured a polyethylene component that is inserted into the tibial plateau. It was designed to be nestled in cancellous bone within the cortical rim while in many current designs the tibial component is meant to rest on the cortical bone such as the prosthesis that is now being described. Like the prosthesis of "The use of Hemiarthroplasty Prosthesis for Advanced Osteoarthritic and Rheumatoid arthritis of the knee," by MacIntosh, D. I. and Hunter, G. A. (J. Bone Joint Surgery, 54-B(2): 244-255, 1972), the Marmor tibial component had no peg and therefore had no way of having stabilization for the tibial prosthesis "Unicondylar Knee Arthroplasty. 2-10 Year Follow-Up Evaluation," by Cartier, P. and Choaib S. (J. Arthroplasty, 2: 157-162, 1987).
Most implants however had one or two pegs, and one manufacturer had added a small "X" shaped peg. As with a total knee replacement, tibial components can be all polyethylene or metal backed. Some designers have returned to the all polyethylene tibial component while others consider the modular metal back systems to be safer and more beneficial. Unicompartmental tibial components can be inserted with or without cement and are available in modular or one piece designs, or available with a fixed or mobile tray as described in "Mensical-Bearing Unicompartmental Knee Arthroplasty. An 11-year Clinical Study," by Cohen, M., Buechel, F. and Pappas, M. J. (Orthop. Rev., 20:443-448, 1991). U.S. Pat. No. 5,207,711 utilizes both the cement-securing method and other securing devices (screws in that patent) to secure the prosthesis. The '711 patent describes the common problems that arise due to the use of cement, such as misalignment, anchoring problems, and the escape of excess cement that can lead to crumbling (and therefore irritation) and jeopardize the integrity of the cemented parts.
The femoral component of the unicompartmental prosthesis can be made of the resurfacing type which necessitates resection of only a few millimeters of the posterior condyle, or it can be more of a total knee type which necessitates condylar resection such as distal, posterior and chamfer cuts. Femoral components can be inserted with or without cement. A number of unicompartmental replacements look like half of the parent total knee system that are now present and instrumentation is similar as described in "Universal Intramedullary Instrumentation for Unicompartmental Total Knee Arthroplasty," by Bert, J. M. (Clin. Orthop., 271: 79-87, 1991. It should be noted that the reported results of unicompartmental replacements have varied with the type of implant and from surgeon to surgeon. The results are to be examined in light of the operative alternatives and the ease or the difficulty once the prosthesis can be revised or converted to a total knee replacement. "Unicompartmental Tibiofemoral Resurfacing Arthroplasty," by Laskin, R. S. (J. Bone and Joint Surgery, 60-A: 182-185, March 1978) found that only 65 percent of 37 knees had satisfactory relief from pain at two years and "Unicompartmental Knee Arthroplasty. Ten- to 13-year follow-up study," by Marmor, L. (Clin. Orthop., 226: 14-20, 1988) reported to find 21 failures in 60 knees. He thought these failures were due mostly to an excessively thin tibial component, (6 millimeters) and by the standards of today, poor selection of patients. "Unicompartmental Knee Arthroplasty. Eight- to 12-year Follow-Up Evaluation with Survivorship Analysis," by Scott, R. D., Cobb, A. G., McQueary, F. G. and Thornhill, T. S. (Clin. Orthop., 271: 96-100, 1991) however found that in 100 consecutive unicompartmental replacements, 85 percent had survived 10 years and "Unicompartmental Knee Arthroplasty. A Multicenter Investigation with Long-Term Follow-Up Evaluation," by Hock, D. A., Marmor, L., Gibson, A. and Rougraff, B. I. (Clin. Orthop., 286: 154-159, 1993) reported that 91 percent of 294 implants had survived for over 10 years "Unicompartmental Knee Replacement," by Marmor, L. (In Total Knee Arthroplasty, pp. 245-280. Edited by J. A. Rand. New York, Raven Press, 1993). Therefore the prosthesis itself and the concept that unicompartmental knees are widely accepted and is seen as a beneficial adjunct to the treatment of severe arthritis of the knee.